check to see if a particular permission is implied in the
collection, using the implies method.

enumerate all the permissions, using the elements method.

When it is desirable to group together a number of Permission objects
of the same type, the newPermissionCollection method on that
particular type of Permission object should first be called. The default
behavior (from the Permission class) is to simply return null.
Subclasses of class Permission override the method if they need to store
their permissions in a particular PermissionCollection object in order
to provide the correct semantics when the
PermissionCollection.implies method is called.
If a non-null value is returned, that PermissionCollection must be used.
If null is returned, then the caller of newPermissionCollection
is free to store permissions of the
given type in any PermissionCollection they choose
(one that uses a Hashtable, one that uses a Vector, etc).

The PermissionCollection returned by the
Permission.newPermissionCollection
method is a homogeneous collection, which stores only Permission objects
for a given Permission type. A PermissionCollection may also be
heterogenous. For example, Permissions is a PermissionCollection
subclass that represents a collection of PermissionCollections.
That is, its members are each a homogeneous PermissionCollection.
For example, a Permissions object might have a FilePermissionCollection
for all the FilePermission objects, a SocketPermissionCollection for all the
SocketPermission objects, and so on. Its add method adds a
permission to the appropriate collection.

Whenever a permission is added to a heterogeneous PermissionCollection
such as Permissions, and the PermissionCollection doesn't yet contain a
PermissionCollection of the specified permission's type, the
PermissionCollection should call
the newPermissionCollection method on the permission's class
to see if it requires a special PermissionCollection. If
newPermissionCollection
returns null, the PermissionCollection
is free to store the permission in any type of PermissionCollection it
desires (one using a Hastable, one using a Vector, etc.). For example,
the Permissions object uses a default PermissionCollection implementation
that stores the permission objects in a Hashtable.

Subclass implementations of PermissionCollection should assume
that they may be called simultaneously from multiple threads,
and therefore should be synchronized properly. Furthermore,
Enumerations returned via the elements method are
not fail-fast. Modifications to a collection should not be
performed while enumerating over that collection.

toString

Returns a string describing this PermissionCollection object,
providing information about all the permissions it contains.
The format is:

super.toString() (
// enumerate all the Permission
// objects and call toString() on them,
// one per line..
)

super.toString is a call to the toString
method of this
object's superclass, which is Object. The result is
this PermissionCollection's type name followed by this object's
hashcode, thus enabling clients to differentiate different
PermissionCollections object, even if they contain the same permissions.

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